Note: Descriptions are shown in the official language in which they were submitted.
?5~35~
A process based on circulating fluidized bed
technique for cooling gases, and a circulating
fluidized bed cooler used in the process
The pxesent invention relates to a process for
cooling gases using a circulating fluidized bed
technique, wherein gas is conveyed through a
fluidized bed formed by circulating fluidizecl bed
matarial so that part of the circulating fluidized
bed material is driven along with the gas flow and
whereby the circulating fluidized bed material
entrained in the gas flow is separated from the gas
flow and returned into the fluidized bed, and wherein
at least part of the cooling is carried out by
cooling the circulating fluidized bed material stream
to be returned. The invention also relates to a
circulating fluidized bed cooler for cooling gas,
which comprises means for feeding the gas to be
cooled into the cooler, a discharge duct for venting
the cooled gas, particulate circulating fluidized bed
material forming a fluidized bed in the lower portion
of the cooler and flowing along with the ga~ to the
discharge duct, at least one return duct for
returning the circulating fluidized bed material to
the fluidized bed, and at least one cooling member
for cooling the gas stream and the circulating
fluidized bed material,
The circulating fluidized bed technique is
commonly implemented in combustion and gassing
processes. The essential advantage of the circulating
fluidized bed technique in comparison with other
reaction types is an excellent material and heat
transfer between the particles and the gas. By using
a sufficient gas velocity, a nearly isothermic state
is produced in the reactor. This essentially
~acilitates the managing of the combustion and gas~
sing processes.
Also Finnish Patent Application No. 813717 de-
saribes an embodiment of the circulatin~ fluidized
bed technique in the recovery oE hea~ from gases
containing mol~en material and/or vapours. In the
solution disclosed in this reference, the procsss gas
is cooled by means of cooling members directly
cooling the gas flow, and the circulating fluidized
bed material entrained in the gas flow is returned to
the fluidized bed, into a duct leading from the gas
discharge duct to the side of the *luidized bed. This
solution has the drawback that the regulation of the
gas temperature, and particularly when one desires a
certain discharge temperature, of the operation of
the coolers located directly in the gas flow, is not
sufficiently effective, and thus the operative range
of the apparatus is rather narrow. Further, the cool-
ing of the particles entrained in the gas flow in
such a cooler is not suffici~ntly effective, where-
fore they are returned to the fluidized bed rela-
~ively hot, which further impairs the operation of
the apparatus.
From the point of view of the circulating
fluidized bed cooler, it is essential that the
circulating fluidized bed material flow can be
regulated independently of the rated load of the
apparatus. Further, in many processes it is desirable
that the temperature of the reactor remains constant
within a wide load range. This is o~ particular
importance in embodiments in which the chemical
kinetics requires operation in a narrow t~mperature
range. With the known implementations of the
circulating fluidized bed technique, this is not
possible.
" 2~:5~3~55
It is known per æe to cool the return circu-
lation of a recycling particle stream. For instance
in U.S. Patent No. 4,165,717, a separate heat exchan-
ger based on a bubbling fluidized bed technique has
been used in the return circulation. Then e.g. a sep-
ara-te fluidizing gas stream must be used, which has a
disadvantageous effect on the operation of the cyc~
lone. Also, the solution is complicated to realize in
practice, and its process regulation is difficult to
manage. In some cases, it has been practical in other
respacts to construct the return duct to be cooled,
in which case usually a small extent of the cooling
has been effected in the return duct. These solutions
are both complicated and dif~icult to realize, and
also their controllability is rather low and their
range o~ operation narrow.
The ob;ect of the present invention is to pro-
vide a process and a circulating fluidized bed cooler
wherein the exhaust temperature of the gas stream to
be treated can be regula-ted within wide limits
independently of the load on the apparatus. The
process of the invention is characterized in that
substantially the entire cooling is realized by
coolin~ the circulating fluidized bed material stream
to be returned in a hea-t exchanger separate from the
gas flow duct by means of a cooling medium separate
from the circulating fluidized bed material, in which
event the cooling of the gas stream is effected by
means of a circulating fluidized bed material stream
returned to the ~luidized bed and having a
temperature lower than that of the fluidized bed. The
circulating fluidized bed cooler of the invention i~
characterized in that it comprises one heat exchanger
serving as a cooling member, through which a cooling
medium is conveyed, that all return ducts for
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returning the circulating fluidized bed material to
the fluidized bed lead -through the heat exchanger so
that the returned circula~ing fluidized bed material
is cooled in the heat exchanger separately from -the
cooling medium, that a closure spaoe with an aperture
is provided below the coolex, in w:hich closure space
the returned circulating fluidized bed material con-
tinually forms a plug-like layer, thus preventing the
flow of gas through the return ducts into the gas
exhaust duct.
Tha essential feature of the cooling method of
the present invention is that -the majority of the gas
cooling (preferably 80-100% of the total cooling) is
effected by cooling the solids in a cooler disposed
substantially vertically in the return duct for
solids, the solids flowing freely through said cooler
on the action of gravitational acceleration, and by
cooling the gas by means of the cooled returned
solids. This object is achieved in such a way that
the maiori~y of the cooling surfaces are disposed in
the solids return stream of the circulating fluidized
bed cooler, in which event the gas to be cooled is
substantially not in contact with the coollng
surfaces, but the cooling of the gas is effected by
mixing the particles that have cooled in the heat
exchanger connected to the circulating fluidized bed
material return duct with the gas to be treated. In
accordance with the invention, the heat exchanger
preferably has a number of return ducts or tubes, via
which the circulating fluidized bed material, such as
sand, can flow through the heat exchanger downwardly,
the heat e~changer serving as a cooling member as air
or some other suitable medium flows therethrough via
the adjacent ducts, said medium being passed in the
exterior of the return ducts, cooling their walls and
Z~5~3S55
thereby also the circulating fluidized bed material
to ba returned, such as sand, simultaneously being
warmed up itself. An advantage of the process accord
ing to the present in~ention is that the regulation
of the cooling efficiency is easy and simple to per-
form, since the cooling is mainly conducted indir-
ectly by cooling the circulating fluidized bed
material, and at the most a small extent of the
cooling is performed directly in such a way that the
process gas flows along the heat-conducting outer
surface of the cooler.
In the following, the invention will be ex-
plained in more detail with reference to the accom-
panying drawings, wherein
Figure 1 shows the principle of the construc-
tion and the operation of the circulating fluidized
bed cooler of the invention,
Figure 2 shows a cross-section of the cir-
culating fluidized bed cooler of Figure 1 along line
A - A, and
Figures 3a and 3b show in more detail the con-
struction and mode of operation of the jetting bot-
tom of the circulating fluidized bed cooler.
Figure 1 shows a circulating fluidized bed
cooler according to the invention. It comprises a hot
gas reactor 1, wherefrom the gas to be cooled is
conducted to the circulating fluidized bed cooler.
Reference numeral 2 denotes a recycle gas grate
including fluidizing nozzles 13 and 13a (to be
explained hereinbelow~, wherefrom the recycle gas is
conducted to the fluidized bed. The recycle gas grate
2 also includes hot gas nozzles indicated with
reference numeral 3. The lower portion of the
circulating fluidized bed cooler, i.e. the fluidized
bed chamber, has been indicated with reference
2~5~355~i
numeral 4. To ensure a sufficient particle
concentration, the cross-section o~ the lower portion
4 is greater than that of the annular upper portion,
that is, of the reaction chamber 5 through which the
gas and particles flow into the upper portion of the
circulating fluidized bed coole~r.
From the annular reaction chamber 5, the gases
and the circulating fluidized bed material particles
are conveyed tangentially to a cyclone chamber 9
through a cyclone vane gra~e 6 disposed axial-
symmetricaIly in the upper portion of the circulating
fluidized bed coolerO The gas substantially purified
of particles is discharged through the central tube
of the cyclone, i.e. exhaust tube 7, for further pro-
cessing. An intermediate floor disposed within thecyclone has been indicated with reference numeral 8,
which separates the cyclone chamber 9 and the
particle distribution chamber 15 therebelow. The
purpose of the distribution chamber is to distribute
the particle stream migrating into the return
circulation, i.e. to bè returned to the fluidized
bed, evenly between the return ducts or -tubes of the
typically recuperative heat exchanger 10. A nozzle
piece 14 has been installed in the apparatus to form
a closure chamber in which the circulating fluidized
bed material returned -through the heat exchanger 10
forms a plug-like layer, flowing evenly through the
aperture at the centre of the nozzle piece 14 into
the fluidized bed chamber 4. This is intended for
preventing the flow of gas upwardly through the heat
e~changer 10. The inlet passage for the cooling
stream of gas or other medium into the heat exchanger
10 has been indicated with reference numeral 11, and
the outlet passage of said stream out from the heat
exchanger has been indicated wi~h reference numeral
- , . ; .
.
12. The stream of cooling medium passes through the
haat exchanger in the sxterior of the return ducts,
cooling tha ducts and thereby the particle stream
contained therein, simultaneously being warmed up
itself.
Figure 2 is a schematic cross-sectional view of
the circulating fluidized bed cooler of Figure
cross-cut along line A - A. Figure 2 shows an annular
reaction chamber 5 at the centre of which a
recuperative cooler 10 has been disposed
substantially symmetrically. Return ducts lOa
extending from the chamber 15 up to the space of the
nozzle piece 14 lead through the cooler 10. A cooling
medium, which may be air, some other suitable gas,
water, some other liquid or steam depending on the
application used, flows in the e~terior of the ducts
lOa.
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2~3S5~
Example
_ t gas Inlet utlet
Temperature 960C 480C
5 Pressure 1.1 MPa 1.0 MPa
Gas stream 86.8 mol/s 86.8 mol/s
Stream of Na2S 0.37 mol/s 0.37 mol/s
Stream of Na2CO3 0.88 mol/s 0.83 mol/s
10 Coolin~_air Inlet Outlet
Temperature 290C 416C
Pressure 1.3 MPa 1.2 MPa
Gas stream 5410 mol/s 5410 mol/s
The particle concentration in the lower portion
of the circulating fluidized bed cooler is prefer~
ably 50-150 kg/m3 and in the upper portion 5-30 kg/m3.
By using a recycle material having a suitable size
distribution, the particle density profile can be
adjusted within said percentage limits independently
of the gas stream. A typical gas velocity affording
an apparatus-technically practical construction
varies in the range 3-8 m/s. If the particle size
distribution of the recycle material cannot be
selected, one must in certain cases use considerably
higher velocities, even up to 30 m/s.
In accordance with the lnvention, a sufficient
circulating fluidized bed material stream is
maintained by means of a recycle gas which is
conveyed into the circulating fluidized bed cooler
through a separate jetting bottom 16 shown in Figures
3a and 3b from nozzles 13 so that it maintains the
zone between the hot gas nozzles 3 in a fluidized
state. Figure 3b shows a jetting bottom accordiny to
Figure 3a cut open along the line B - B, showing the
2~5~55~5
jetting bottom construction of the invention and its
operation. In accordance with the present invention,
the circulating fluidi2ed bed cooler thus advan-
tageously has a double Jetting bottom wherein the hot
gas to be cooled is conveyed ~nto the cooler through
nozzles 3 preferably of a relative]y large size (D =
20-60 mm), and the recycle gas i9 convayed through
conventional smaller nozzles 13. Further, the fluid-
izing gas is conveyed through nozzle apertures 3,
which are provided in the lower plate 16a of the
double jetting bottom 16 and which convey process
gas, about the incoming stream of process gas in such
a way that the upper jetting bottom 16b above the
process gas apertures 3 has apertures 13a of a larger
diameter than apertures 3 at the location of said
process gas apertures, so that the fluidizing gas B
issuing through duct 2 flows between the lower and
upper jetting bottom 16a and 16b and is pushed about
the process gas flow A into an annular form, in addi-
tion to its flowing through the apertures of thenozzles. With this arrangement, one can avoid incrus-
tation about the nozzles for hot gas, which other-
wise often presents a problem if the hot gas contains
condensing compounds or, on account of its high tem
perature, tends to melt the fluidized bed material
about the nozzles.
In certain cases, it has been practical to add
to or maintain a high particle concentration only in
a shallow zone above the jetting bottom. In that
event, one can for instance prevent the formation of
collars about the hot gas nozzles without increasing
the solids stream passing through the cooler. Also
molten and condensing material which is possibly
present can then be collected substantially about
said coarse material. The circulating fluidized bed
2~?5~5~5
material stream is maintained by maans of a more
finely divided particulate material. By regulating
the recycle gas flow, one can regulate the material
stream through the heat exchanger connected to the
circulating fluidized bed materia:L stream, in which
sitllation also the heat exchanging capacity of the
cooler is regulated~ Since upon the diminishing of
the recycle gas flow a considerable portion of the
particles remain in the fluidized state on the nozzle
bottom of the cooler, the influence of the regulation
of the recycle gas flow is rapid and effective. An-
other way to regulate ~he heat exchanging efect is
to add or diminish the amount of circu:Lating
~luidized bed material in the cooler.
In the foregoing description and drawingR, the
invention has been explained only by way of example,
and it is in no way restricted to said example.
